The invention relates to a heavy-duty vehicle tire and more specifically to the tread of such a tire, regardless of the characteristics of the tread.
In the vast majority of cases, the tread patterns on treads for heavy-duty vehicle tires consist of straight, zigzag or wavy circumferential grooves, said grooves sometimes being interconnected by transverse grooves and/or incisions. Said circumferential grooves generally have wear indicators, which are small platforms of rubber covering the bottom of the grooves over a certain circumferential length, said indicator indicating the minimum depth of tread that legally has to remain on the tread in use. Treads for heavy-duty vehicles are regroovable, and tires with such tread patterns have the term "regroovable" or the symbol "U" marked on their sidewalls. Regrooving, on the one hand, allows the adherence potential of the heavy-duty vehicle tire to be extended and, on the other hand, the tire life (expressed in kilometers) to be significantly improved by 15 to 30%, depending on the case, and does so without penalizing the possibility of retreading, which incidentally is an essential feature of a heavy-duty vehicle tire. It should be added that regrooving also gives a fuel saving, as the tire has a lower rolling resistance because of the reduced thickness of the tread.
As is known per se, regrooving a groove is done using a rounded heated blade manipulated by an operator. Said blade, connected to a support which rests on the surface of the tread, can be manipulated manually to make it follow the line of the groove on the surface of the tread, quite accurately, even in the case of a groove that does not follow a straight path. This regrooving operation does, however, demand a number of precautions. The first consists in said operation being carried out when about 2 mm of groove depth remains, said depth being measured between the surface of the tread and the radially outer surface of the wear indicators placed at the bottom of the groove. This precaution allows the tread pattern to be properly seen and thus reproduced without major difficulties. Knowing the remaining depth of the tread and the depth of recut recommended by the tire manufacturer, it is then possible to adjust and set the height of the regrooving blade.
The regrooving depths generally recommended are theoretical depths. Although in most cases these are satisfactory and theoretically allow the blade height to be set so that, generally speaking, a certain thickness of rubber is left between the bottom of the recut groove and the radially upper face of the crown reinforcement, the risks of regrooving too deeply are not precluded. Now, excessive regrooving can cause damage that leads to the premature destruction of the tire casing. It can also compromise the possibility of economic retreading, that is to say retreading in which just the tread is changed. It can also, in some extreme cases, make the plies of the radially-underlying crown reinforcement visible at the bottoms of the regrooved tread, and this is something which is not generally permitted by the legislation in force.